The present disclosure is generally directed to biosensors including channels of functionalized graphene or functionalized carbon nanotube surfaces configured to immobilize to the surface analytes such as metal particles, polymers, organic molecules, macromolecules and particularly of biological molecules chosen from peptides, polypeptides, proteins such as enzymes, nucleic acids, antibodies or antibody fragments, polysaccharides, cells and cell fragments.
Graphene and carbon nanotubes (CNTs) have attracted attention in the field of sensing due to their exceptional charge transport characteristics, which are generally confined to the surface and are able to detect molecular level changes in their immediate environment since these materials. Ever since the first demonstration of CNT sensing capability on gas molecules, numerous studies have reported on the interaction of CNTs with a variety of biological and bioactive species such as proteins, peptides, DNA, enzymes, and the ability to transduce this interaction into an effective sensor. By virtue of their small size, the sensitivity of the detecting or sensing elements such as nanotubes and nanowires, and the versatility to detect specific bindings of a vast group of analyte: analyte-binding molecules, nanostructure-based biosensors are rapidly gaining employability in real-time detection of the presence of biological molecules. For instance, single-walled carbon nanotubes (SWCNTs) have been employed in the detection of deoxyribonucleic acid (DNA) based on both electrochemical and transistor configuration and detection limits of the order of parts per trillion have been reported.
Graphene is generally described as an open nanotube of a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a truly planar graphene structure does not exist in a three-dimensional space, being unstable with respect to formation of curved structures such as soot, fullerenes, nanotubes or buckled two dimensional structures. However, a two-dimensional graphene structure may be stable when supported on a substrate, for example, on the surface of a silicon carbide (SiC) crystal. Free standing graphene films have also been produced, but they may not have the idealized flat geometry.
Structurally, graphene has hybrid orbitals formed by sp2 hybridization. In the sp2 hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp2 orbitals. The one remaining p-orbital forms a pi (π)-bond between the carbon atoms. Similar to the structure of benzene, the structure of graphene has a conjugated ring of the p-orbitals, i.e., the graphene structure is aromatic. Unlike other allotropes of carbon such as diamond, amorphous carbon, carbon nano foam, or fullerenes, graphene is only one atomic layer thin.
Graphene has an unusual band structure in which conical electron and hole pockets meet only at the K-points of the Brillouin zone in momentum space. The energy of the charge carriers, i.e., electrons or holes, has a linear dependence on the momentum of the carriers. As a consequence, the carriers behave as relativistic Dirac-Fermions with a zero effective mass and are governed by Dirac's equation. Graphene sheets may have a large carrier mobility of greater than 200,000 cm2/V-sec at 4K. Even at 300K, the carrier mobility can be as high as 15,000 cm2/V-sec.
Graphene layers may be grown by solid-state graphitization, i.e., by sublimating silicon atoms from a surface of a silicon carbide crystal, such as the (0001) surface. At about 1,150° C., a complex pattern of surface reconstruction begins to appear at an initial stage of graphitization. Typically, a higher temperature is needed to form a graphene layer. Graphene layers on another material are also known in the art. For example, single or several layers of graphene may be formed on a metal surface, such as copper and nickel, by chemical deposition of carbon atoms from a carbon-rich precursor.
Graphene displays many other advantageous electrical properties such as electronic coherence at near room temperature and quantum interference effects. Ballistic transport properties in small scale structures are also expected in graphene layers.
While single-layer graphene sheet has a zero band-gap with linear energy-momentum relation for carriers, two-layer graphene, i.e. bi-layer graphene, exhibits drastically different electronic properties, in which a band gap may be created under special conditions. In a bi-layer graphene, two graphene sheets are stacked on each other with a normal stacking distance of roughly 3.35 angstrom, and the second layer is rotated with respect to the first layer by 60 degree. This stacking structure is the so-called A-B Bernel stacking, and is also the graphene structure found in natural graphite. Similar to single-layer graphene, bi-layer graphene has zero-band gap in its natural state. However, by subjecting the bi-layer graphene to an electric field, a charge imbalance can be induced between the two layers, and this will lead to a different band structure with a band gap proportional to the charge imbalance.
Because of the unique electric properties associated with carbon nanotubes and graphene, these materials are attractive for applications in nanotechnology. Semiconducting carbon nanotubes, in particular, have received attention, due to their promising performance in electronic devices, such as diodes and transistors. For example, carbon nanotubes and graphene can be used as channels in field effect transistors (FETs). The most common prior art method of fabricating carbon nanotube FETs starts with depositing a carbon nanotube on a thin oxide film from a liquid suspension. Source and drain contacts are then formed lithographically on the nanotube to form a FET device.
The deposition of carbon nanotubes on an oxide surface, followed by lithographic patterning of the source and drain contacts, has been successfully used in the prior art for the construction of single carbon nanotube FETs. However, fabrication of integrated circuits from nanotubes requires the precise placement and alignment of large numbers of carbon nanotubes on a surface (e.g., spanning the source and drain contacts). E. Valentin, et al., “High-density selective placement methods for carbon nanotubes”, Microelectronic Engineering, 61-62 (2002), pp. 491-496 disclose a method in which the adhesion of carbon nanotubes onto a SiO2 surface is improved using aminopropyltriethoxysilane (APTS). In this prior art, APTS is employed to form a silanized surface on SiO2, which is then used to selectively place the carbon nanotubes.
A drawback with the prior art process disclosed in the E. Valentin, et al. article is that the trialkoxysilane undergoes polymerization in solution and self-assembly must be carried out under controlled conditions excluding water. Additionally, APTS cannot be printed using conventional poly(dimethylsiloxane) (PDMS) stamps in contact printing because the solvents that are used for APTS could swell and destroy such stamps.
Diazonium resins and salts have also been used for forming multiple layers of enzymes or of polyoxometallates on the surface of various materials by electronic complexation and then. This process remains limited since it requires the existence of electronic interactions between the species intended to be deposited on the surface and the diazo-compounds. In addition, it does not appear to allow grafting with the surface. Moreover, diazonium salts are not very stable compounds and upon exposure to ambient light and/or moderately high temperatures, these materials have been known to dissociate resulting in removal from the surface.